The present invention relates generally to the construction, configuration, and use of an electrical machine comprising a motor having an integral detent or magnetic brake. The motor is particularly adapted for use within an actuation system for driving a leading edge flap of an aircraft airfoil which requires periodic holding against substantial back-driving forces on the flap.
Electrically driven primary flight control surfaces such as leading edge flaps require a detent or brake to hold motor position against aerodynamic loads during flight which tend to back-drive the actuation system. Previous designs have used motor stall current to provide the brake function. However, this requires continuous energization of one motor winding resulting in unacceptable thermal stress. The thermal stresses and the continuous energization of a winding combine to substantially reduce the reliability of the motor. In addition, for dual electric motor driven actuation systems, an additional friction type brake is required to provide a reaction point for single motor operation. Present state of the art systems utilize solenoid operated friction type brakes that are physically large and subject to wear, requiring periodic maintenance and replacement.
Because primary flight control surfaces, such as leading edge flap drives, are required to be highly reliable, designers have generally used hydraulic motor drive systems instead of electric motor drive systems. However, hydraulic systems in aircraft have their own set of limitations and reliability problems. Accordingly, it is desirable to have a highly reliable electric motor drive system as an alternative to hydraulic systems.
In U.S. Pat. No. 4,852,245, a parent application to the present invention, Applicant details a high reliability, high power density toothless stator motor which is relatively inexpensive to produce and which eliminates the usual "T" shaped ferromagnetic stator core teeth. The copper windings are installed in slot areas between adjacent radially outwardly extending support fins of a plastic cylindrical winding support structure. The support fins and the winding support structure do not carry magnetic flux and are relatively thin, thereby the slot area in which the stator windings are installed is maximized. The stator windings may be pre-wound on a form and easily dropped into the slots between the support fins. A laminated cylindrical flux core surrounds the stator windings and support structure to provide a magnetic flux return path for the rotor.
High energy product permanent magnets having significant energy product increases over previously known permanent magnets allow the construction of a high strength permanent magnet rotor for use with the above described toothless stator motor. For example, samarium cobalt permanent magnets having an energy product of thirty mega-gauss-oersted (MGO), or neodymium-iron-boron magnets which have an energy product of thirty-five MGO are now available. A rotor making the maximum use of high energy product permanent magnets is disclosed in Applicant's U.S. Pat. No. 4,667,123 issued May 19, 1987. The use of such high energy product permanent magnets permits reliable electric machines to be built which are capable of supplying high power outputs.
While the above references detail an electric motor which has the requisite power and reliability for use in a primary flight surface drive actuation system, the brake problem remained a limiting factor. Accordingly, a high power density electric motor which includes a highly reliable brake is very desirable.